Intravascular stents have long been applied to maintain vascular patency. Intravascular stents are used in conjunction with balloon angioplasty wherein a balloon is inflated to expand a constricted vessel in order to restore proper blood flow. The intravascular stent is then positioned inside the now expanded vessel to ensure the vessel maintains the enlarged diameter.
However, attempts to develop a prosthetic stent which would hold open a blood vessel and not develop transluminal thrombus have enjoyed limited long term success. There has been very little significant improvement with the exception of an effort to create a more expansible metallic stent.
For a metallic stent to satisfy the limits for antithrombogenesis while simultaneously maintaining the lumen of a blood vessel in which the stent has been placed, the stent has to fulfill the electrochemical laws for thrombosis. That is, the stent has to maintain a potential difference more negative than plus 250 millivolts versus the normal hydrogen electrode. In addition, the stent must exhibit limited corrosion and limited tissue destruction over the duration of the stent life. It was found very early on that while some of the metals on the corrosive side of the electromotive series would maintain a very negative potential, many of these metals upon ionizing and going into solution produced cellular destruction due to tissue and cellular toxicity. For this reason, the number of materials which can be used to develop a metallic implantable intravascular stent is limited to four or five metals that are known to be anti-thrombogenic and anti-corrosive. The most useful of these appears to be titanium and aluminum.
Titanium and aluminum produce a non-soluble surface oxide on exposure to blood and tend not to go into solution. In addition, titanium and aluminum develop a very negative potential with reference to the normal hydrogen electrode. Titanium and aluminum deposit almost no coagulant materials, coagulant enzymes, or proteins.
A number of patents have been found describing various stent designs as well as methods for delivery of the stent to the desired position in the vessel. These patents include:
U.S. Pat. Nos. 3,868,956 and 4,503,569, each of which describes methods wherein a stent comprising a temperature responsive device is implanted in the damaged vessel and thereafter expanded by means of an external heat source.
U.S. Pat. No. 4,553,545, which discloses a method whereby a complex mechanical rotating device and coaxial cables are employed to increase the diameter of the implanted stent.
U.S. Pat. No. 4,580,568, which describes a stent wherein a single wire forming a closed loop is expanded in the damaged vessel to maintain vascular patency. The loop of wire is compressed to form a series of straight segments and bends, wherein the bends store energy in the compressed state. Upon removal of a compression means the stent expands and exhibits a circular configuration.
U.S. Pat. No. 4,649,992, which describes a device in combination with a catheter which is a compression spring retained by a partially inflated balloon and an abutment immediately behind the balloon on the catheter shaft. The spring prosthesis is transported in this manner to the desired location and released by totally evacuating the balloon thereby allowing the spring prosthesis to expand linearly.
U.S. Pat. No. 4,681,110, which describes a catheter for delivery of a stent comprising woven plastic strands forming a tube which can be compressed radially. The orientation of the plastic strands provide the resilience for tube to expand from the compressed state.
U.S. Pat. No. 4,768,507, which discloses a catheter comprising an outer cylinder and inner core, wherein said inner core comprises spiral grooves for containing a coil spring stent. Pliers are used to facilitate the loading of the coil spring into said grooves whereupon completion of the loading of the outer cylinder is placed over the inner core thereby retaining the coil in they compressed state until the coil is released.
U.S. Pat. Nos. 4,690,684, and 4,720,176, each of which discloses a stent for aligning the ends of the vessel during anastomosis by thermal bonding. The stent comprises an integral solid of biologically compatible material to align the vessel ends together during anastomosis. Upon completion of the anastomosis the stent fully melts into the fluid flowing through the vessel. U.S. Pat. No. 4,770,176 also discloses a method of anastomosing a vessel utilizing the stent described in U.S. Pat. No. 4,690,684.
U.S. Pat. No. 4,878,906, which describes a prosthesis comprising a flexible thin-walled plastic sleeve for repairing damaged vessels. The sleeve having sufficient length to cover the damages area of the vessel forms a sealed interface on its outer peripheral ends with the inner peripheral surface of the vessel, thereby providing a bridge, bypassing the damaged area of the vessel.
U.S. Pat. No. 4,830,003, which discloses a cylindrical shaped stent comprising angled wires of biocompatible metal. The angled wires are connected obliquely at alternate ends to form a compressible open ended tube.
U.S. Pat. No. 4,866,062, which discloses a radially expandable coronary stent. The stent comprises a flat expandable wire band which is preformed in a zigzag pattern to provide expansion capability. The band which is wound into a cylindrical shape is inflated by means of a variable diameter device. The band expands radially exhibiting a cylindrical shape with increasing diameter.
U.S. Pat. Nos. 4,800,882, 4,739,762 and 4,733,665, each of which discloses an expandable intraluminal graft. These grafts are made of wire or a thin walled tubular member and can be expanded by an angioplasty balloon associated with a catheter.
U.S. Pat. No. 4,760,849, which discloses a planar blank which may be made into a helical coil spring stent.
U.S. Pat. No. 4,665,918, which describes a system and method for implanting a generally tubular prosthesis member having an unobstructed central passageway into the length of a blood vessel. The prosthesis member contracts to a smaller dimension for delivery through the unobstructed portion of the blood vessel, and is outwardly expansible in the blood vessel. The prosthesis member is positioned in a contracted condition between a delivery catheter and outer sheath, and expands outwardly in response to the removal of the sheath.
None of the aforementioned patents, however, disclose an anti-thrombogenic stent which decreases turbulence and improves hydraulic flow of blood therethrough. Accordingly, there remains a need for such a device.